New and advanced transmission systems and braking systems are being developed by the automotive industry. These new systems often involve high energy requirements. Therefore, the friction materials technology must also be developed to meet the increasing energy requirements of these advanced systems.
The friction material must have high heat resistance in order to be useful in the new transmission and braking systems. Not only must the friction material remain stable at high temperatures, it must also be able to rapidly dissipate the high heat that is generated during operating conditions.
The high speeds generated during engagement and disengagement of the new transmission and braking systems mean that a friction material must be able to maintain a relatively constant frictional engagement throughout the engagement and disengagement of the friction material. It is important that the frictional engagement be relatively constant over a wide range of speeds and temperatures in order to minimize "grabbing" of materials during braking or "shuddering" of the transmission system during power shift from one gear to another.
In particular, the new high energy friction material must be able to withstand high speeds wherein surface speeds are up to about 12,000 feet/minute. Also, the friction material must be able to withstand high energy pressures up to about 700 psi. It is also important that the friction material be useful under limited lubrication conditions and also be able to withstand the extreme pressures and speeds applied during use.
Previously, asbestos fibers were included in friction materials. For example, the Arledter et al. U.S. Pat. No. 3,270,846 patent describes phenolic and phenolic-modified resins in asbestos-filled friction materials. Now, however, due to health and environmental problems, asbestos is no longer being used. However, friction materials without the presence of asbestos not only encountered structural integrity problems during processing but also lacked the thermal stability which the asbestos provided. More recent friction materials have attempted to overcome the absence of the asbestos in the friction material by impregnating the paper or fiber materials with phenolic-modified and other new resins. These friction materials, however, do not rapidly dissipate the high heat generated, which may contribute to reduced heat resistance and unsatisfactory coefficient of friction performance.
While phenolic resins are conventionally used as an impregnant in wet friction materials for wet clutch applications, the phenolic resins have various limitations. The phenolic resin friction materials do not have the high heat resistance necessary for use with the new high energy transmission systems. In particular, the phenolic resins carbonize at a temperature of about 450.degree. C. which is too low to be useful in high energy applications. In addition, phenolic resins are rigid materials and when the phenolic resins are used in a friction material, uneven lining wear, and separator plate "hot spots" are more likely to result if uniform contact is not obtained.
Attempts to overcome the limitations and drawbacks of phenolic resin impregnated friction materials include the replacement of phenolic resins with other thermosetting impregnating resins. One attempt to produce friction materials involves the modification of a phenolic resin with various synthesized modifications.
In order for friction materials to be useful, the friction material must have a wide variety of acceptable characteristics. The friction material must be resilient (or elastic) yet resistant to compression, abrasion and stress; have high heat resistance and be able to dissipate heat quickly; and, have long lasting, stable and consistent frictional performance. If any of these characteristics are not present, optimum performance of the friction material is not met.
It is also important that a suitable impregnating resin be used with the friction material in order to achieve a high energy application friction material. The wet friction material must possess uniform absorbency and must have good tensile and shear strengths both when saturated with the wet resin during impregnation and when saturated with brake fluid or transmission oil during use. In selected applications, it is important that the friction material have a low density and high porosity such that there is a high fluid absorbency capacity during use. Thus, it is important that the friction material not only be porous, but also be flexible and compressible. The fluids absorbed into the friction material must be capable of being squeezed or released from the friction material quickly under the pressures applied during operation of the brake or transmission. It is also important that the friction material have high thermal conductivity to also help rapidly dissipate the heat generated during operation of the brake or transmission.
In view of the need for a better friction material, and as a result of extensive research, a new friction material with improved characteristics has now been developed. As far as is known, there is no disclosure of a friction material for use in transmission systems wherein the friction material is made by adding a powdered phenolic resin and a silicon nitride to a mixture of paper pulp, forming a paper friction material and then impregnating the phenolic resin-paper friction material with a silicone material.
Until the present invention there has been no disclosure or suggestion that a powdered phenolic resin material and powdered silicon nitride could successfully be blended with raw paper pulp to form a friction paper material, which could then be impregnated with a silicone material to form a friction lining material. On the contrary, previous attempts to use silicone resins in friction materials have not had good acceptance in the friction lining industry. A friction lining that is impregnated or saturated with a silicone resin has typically poor shear strength and delamination resistance. Further, the silicone resin tends to cause the friction lining to be too elastic which then creates undesirable friction characteristics (such as potentially large friction fade). It is not surprising that past friction lining compositions formed with a phenol-formaldehyde or polysiloxane resin have not been used successfully. Such compositions do not have the necessary constant coefficient of friction characteristics and thus fail under high energy and high heat conditions.
Accordingly, it is an object of the present invention to provide an improved friction material with reliable and improved properties compared to those of the prior art.
A further object of this invention is to provide friction materials with high thermal conductivity, porosity and strength.